The present invention relates to a vibration body for cutting and a processing device used favorably in the case of forming a molding die for an optical element and others, and a molding die and an optical element produced by using the aforesaid items.
There is available a technology to cut materials such as carbide and glass which are hard-to-cut materials by vibrating a tip of a cutting tool such as a diamond tool, which is called vibration cutting. In this technology, minute cutting-in is conducted at high speed by a cutting edge of a cutting tool through vibration, and chips generated in this time are scraped out by the cutting edge through vibration, resulting in realization of cutting processes which cause less stress for a cutting tool and a material to be cut (for example, see Patent Documents 1, 2, 3 and 4). Owing to this process of vibration cutting, a critical depth of cut needed for an ordinary cutting of ductile mode is improved to be several times as large as its normal depth, thus, the hard-to-cut materials can be subjected to cutting process at high efficiency.
In such process of vibration cutting of this kind, high speed vibration of 20 kHz or more is usually used, because for improving the efficiency of processing, when a vibration frequency is enhanced, the aforesaid effects are increased and a feed rate for the tool is also enhanced in proportion substantially to the frequency. There is also an advantage that an oscillator or a vibration body excited by the oscillator does not cause an offensive noise, because the aforesaid frequency is beyond a human audible range.
As a method to generate high speed vibration on a cutting edge of a cutting tool, a method has been put to practical use wherein a holding member that holds a tool is excited with a piezoelectric element or a super-magnetostrictor, to vibrate stably as a standing wave, by resonating this holding member with bending vibration and axial vibration (axial direction vibration). In this method, the vibration body that holds the cutting tool usually has a columnar outer form having a circular cross section, and its tip portion is tapered off.
However, in the case of using the columnar vibration body such as one mentioned above, when processing an optical surface having a large facing angle, a form error on the surface processed arrives at several microns, and improvement of processing accuracy is not easy, though an optical surface having a small facing angle can be processed relatively accurately. Incidentally, a facing angle means an angle of inclination of a normal line on the optical surface relative to an axis of the vibration body or to the optical axis.
The reason why an improvement of processing accuracy is not easy is considered that control of the state of vibration concerning to bending vibration is not easy because of a circular cross section of the vibration body, and great bending vibration is also generated in the direction perpendicular to the direction of aimed bending vibration. Further, it is also considered that bending rigidity in the direction perpendicular to the direction of aimed bending vibration, because of a circular cross section of the vibration body, and when the reactive force resulting from cutting is applied in the aforesaid direction, a tip portion of the vibration body is displaced relatively simply, thus, a depth of cut of a cutting edge of a tool is changed, and processing accuracy is declined.
(Patent Document 1) Unexampled Japanese Patent Application Publication No. 2000-52101
(Patent Document 2) Unexampled Japanese Patent Application Publication No. 2000-218401
(Patent Document 3) Unexampled Japanese Patent Application Publication No. Hei-9-309001
(Patent Document 4) Unexampled Japanese Patent Application Publication No. 2002-126901